Species Networks

Species Grouping

To correlate species distribution changes with changes in license holdings in Maine, license types have been grouped by target species as described by Maine State Legislation.

License Code	Description	Target Species
al	Aquaculture License: Established in 2017	Aquaculture
cfc	Commercial fishing crew	Finfish
cfs	Commercial fishing single	Finfish
cpc	Commercial pelagic and anadromous crew	Pelagic/Anadromous
cps	Commercial pelagic and anadromous single	Pelagic/Anadromous
cs	Commercial Shellfish	Shellfish
csc	Commercial shrimp crew	Shrimp
cso	Commercial shellfish + 70	Shrimp
css	Commerical Shrimp Single	Shrimp
csu	Commercial Shellfish Under 18	Shellfish
csw	Commercial Shrimp Single	Shrimp
csws	Commercial Shrimp Crew	Shrimp
e0	Elver Dip Net	Elver
e0c	Elver Dip Net Crew	Elver
e1	Elver 1 Fyke Net	Elver
e1c	Elver 1 Fyke Net Crew	Elver
e2	Elver 2 Fyke Net	Elver
e29	Elver Non Resident	Elver
e2c	Elver 2 Fyke Net with Crew	Elver
e6	Elver Dip Net 1 Fyke Net	Elver
e6c	Elver Dip Net 1 Fyke Net Crew	Elver
eel	Eel license - changed to Eel Pot	Eel
eex	Elver Exporter	Elver
elv	Elver	Elver
ep	eel pot/hoop net	Eel
fhc	federal halibut crew	Halibut
gc	Green crab	Green crab
gcn	green crab non resident	Green crab
la	Lobster Crab Apprentice	Lobster
lao	Lobster Crab Apprentice + 70	Lobster
lau	lobster crab apprentace under 18	Lobster
lc1	Lobster crab class I	Lobster
lc2	Lobster crab class II	Lobster
lc2o	Lobster crab class II + 70	Lobster
lc3	Lobster crab class III	Lobster
lc3o	Lobster crab class III + 70	Lobster
lco	Lobster Crab + 70	Lobster
lcs	Lobster Crab Student - policy change in 2016	Lobster
lcu	Lobster Crab Under 18	Lobster
lnc	Lobster crab non commerical	Lobster
mb	Mussel Boat	Mussel
md	Mussel Dragger	Mussel
menc	menhaden commercial	Menhaden
mennr	menhaden commercial non-res	Menhaden
menr	menhaden non commercial	Menhaden
mh	Mussel Hand	Mussel
mwd	Marine Worm Digging	Marine worm
ncf	commercial fishing non resident	Finfish
ncp	non-resident pelagic and anadromous	Pelagic/Anadromous
ncs	Scallop non commericial	Scallop
nfhc	non federal halibut crew	Halibut
nfhs	non federal halibut single	Halibut
nla	Non-res lobster apprentice	Lobster
nlc1	Non-res lobster class 1	Lobster
nrlp	Non-res lob crab landing	Lobster
nrs	Seaweed non resident	Seaweed
nrss	seaweed non resident supp	Seaweed
ntp	non-resident tuna permit	Tuna
qm	Quahog (Mahogany)	Quahog clam
sb	Scallop boat - from 1990 to 1993 - then went to SB	Scallop
scd	Sea Cucumber Drag	Sea cucumber
sd	Scallop Dragger	Scallop
sdi	Scallop diver	Scallop
sh	Scallop Hand - now SDI changed in 1994	Scallop
spt	spat	Spat
sub	Sea Urchin Dragger	Sea urchin
suh	Sea urchin diver	Sea urchin
sur	Sea Urchin Raking	Sea urchin
surf	Surf Clam Boat	Surf clam
sw	Seaweed	Seaweed
swbn	Nonresident Seaweed Buyer	Seaweed
sws	seaweed supplemental	Seaweed

License Portfolio by Species

Using Theresa Burnham’s code for building the individual license portfolio, license types were grouped by target species. A binary matrix was created, whereas 1 denotes an individual license holding for a corresponding year and 0 denotes no license held. With this co-occurrence matrix, we can quantify which license types were held together over the past 30 years and how those holdings have changed.

Species Proportions

Species Matrix

Species	Aquaculture	Eel	Elver	Finfish	Green crab	Halibut	Lobster	Marine worm	Menhaden	Mussel	Pelagic/Anadromous	Quahog clam	Scallop	Sea cucumber	Sea urchin	Seaweed	Shellfish	Shrimp	Spat	Surf clam	Tuna
Aquaculture	0	3	5	33	43	0	293	13	29	23	17	1	34	0	9	26	149	15	40	0	0
Eel	3	0	173	270	74	0	260	60	11	55	38	35	72	0	58	88	140	45	0	13	0
Elver	5	173	0	1793	121	1	4646	1067	65	147	95	36	1261	0	1373	194	3206	326	0	0	0
Finfish	33	270	1793	0	463	17	29510	1758	10145	670	5480975	652	10182	54	6093	1206	6205	4968	21	8	2
Green crab	43	74	121	463	0	0	754	254	39	64	80	42	258	5	115	169	656	134	5	6	0
Halibut	0	0	1	17	0	0	21	0	2	0	5	0	3	0	0	0	3	0	0	0	0
Lobster	293	260	4646	29510	754	21	0	3933	2262	982	1732	994	21390	72	12567	1555	16165	6715	66	42	1
Marine worm	13	60	1067	1758	254	0	3933	0	12	93	33	58	976	0	1175	589	5552	155	0	2	0
Menhaden	29	11	65	10145	39	2	2262	12	0	15	5310	24	291	2	53	32	187	4	18	4	0
Mussel	23	55	147	670	64	0	982	93	15	0	44	189	916	18	451	287	465	154	3	14	0
Pelagic/Anadromous	17	38	95	5480975	80	5	1732	33	5310	44	0	45	567	15	133	88	211	394	11	7	0
Quahog clam	1	35	36	652	42	0	994	58	24	189	45	0	1186	8	619	52	257	184	2	15	0
Scallop	34	72	1261	10182	258	3	21390	976	291	916	567	1186	0	114	9837	621	4263	2549	12	17	0
Sea cucumber	0	0	0	54	5	0	72	0	2	18	15	8	114	0	54	0	3	29	0	0	0
Sea urchin	9	58	1373	6093	115	0	12567	1175	53	451	133	619	9837	54	0	606	4156	769	2	0	0
Seaweed	26	88	194	1206	169	0	1555	589	32	287	88	52	621	0	606	0	773	163	4	13	0
Shellfish	149	140	3206	6205	656	3	16165	5552	187	465	211	257	4263	3	4156	773	0	520	9	9	0
Shrimp	15	45	326	4968	134	0	6715	155	4	154	394	184	2549	29	769	163	520	0	0	6	0
Spat	40	0	0	21	5	0	66	0	18	3	11	2	12	0	2	4	9	0	0	0	0
Surf clam	0	13	0	8	6	0	42	2	4	14	7	15	17	0	0	13	9	6	0	0	0
Tuna	0	0	0	2	0	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0

Annual Species Metrics

Degree and betweenness are both ways of measuring centrality.

“The degree of a node is the number of other nodes that single node is connected to.

Important nodes tend to have more connections to other nodes. Highly connected nodes are interpreted to have high degree centrality.”

“[Betweenness] ranks the nodes based on the flow of connections through the network. Importance is demonstrated through high frequency of connection with multiple other nodes. Nodes with high levels of betweenness tend to serve as a bridge for multiple sets of other important nodes.” Source

Lobster is the most abundant license type held in Maine, its high degree of centrality with relatively low betweenness suggests that while lobster has a lot of connections to other license types, those license types that co-occur with lobster don’t co-occur with many other license types.

Eel and quahog have highly variable betweenness with relatively low degree of centrality, suggesting that these license may be held concurrently with more central license types to supplement harvest.

Finfish also has a high degree of centrality and low betweeness.

Annual Network Plots

Similar to plotting the license holdings as a network over time, we plot the target species pertaining to these permits over time. This makes for a more easily interpretable graph, as well as increased potential for correlating license changes to species distribution changes

The thickness of lines across the network reflect the frequency of co-occurrence. Strong connections between all lobster license types and commercial finfish appear consistently over the 33 year data set. However, since the mid-90’s there have been more and more connections occurring at lesser frequencies. The connection of finfish to lobster stays relatively consistent, suggesting that harvesters are diversifying license holdings rather than exiting entirely.

Limited Entry Fisheries

According to Maine State Legislation, there are 6 limited entry species including lobster, which is limited by harvest zone. Most species became limited entry in response to a fishery collapse, while some aim to avoid collapse. The most recent limited entry fishery is menhaden, which was passed by the legislature in 2023.